Demand for secondary batteries as an energy source has been significantly increased as technology development and demand with respect to mobile devices have increased. Among these secondary batteries, lithium secondary batteries having high energy density and high voltage have been commercialized and widely used.
A current collector is coated with a positive electrode active material or negative electrode active material of appropriate thickness, or the active material itself is prepared in the form of a film of appropriate length, and the film is then wound or stacked with an insulating separator to prepare an electrode assembly. Thereafter, the electrode assembly is put in a can or a container similar thereto, and a lithium secondary battery is then prepared by a process of injecting an electrolyte.
A lithium metal oxide is used as the positive electrode active material, and lithium metal, a lithium alloy, crystalline or amorphous carbon, or a carbon composite is used as the negative electrode active material. Also, an electrolyte in a liquid state, particularly, an ion conductive organic liquid electrolyte, in which a salt is dissolved in a non-aqueous organic solvent, has been mainly used as the electrolyte.
However, as interests in energy storage technologies have been increasingly grown, three is a need to develop a secondary battery which may not only be miniaturized, lightweight, and charged and discharged to high capacity, but may also have high-temperature, high-voltage safety. Accordingly, development of a battery using a gel polymer electrolyte composed of a gel polymer, instead of using a liquid electrolyte, has recently received attention.
In general, it is known that battery safety improves in the order of a liquid electrolyte, a gel polymer electrolyte, and a solid polymer electrolyte, but battery performance decreases in the same order.
That is, the gel polymer electrolyte is disadvantageous in that lithium ion conductivity is lower than that of a liquid electrolyte composed only of an electrolyte solution. A method of decreasing the thickness of the gel polymer electrolyte has been proposed to improve the conductivity. However, in this case, there may be limitations in improving the performance and safety of the battery, for example, mechanical strength is decreased and a short circuit between the gel polymer electrolyte and both positive electrode and negative electrode occurs during the preparation of the battery.
Thus, there is a need to develop a gel polymer electrolyte which may improve both performance and safety of the battery.